1. Field of the Invention
The present invention concerns a thermal head used for a thermal printer and it particularly relates to a thermal head capable of providing a thermal head with real edging and improvement for printing quality in a thermal transfer printer.
2. Description of the Related Art
A thermal head mounted on a thermal printer comprises a plurality of heat generating elements arranged linearly on a substrate in which electric current is supplied successively to the heat generating elements selectively based on desired printing information to heat the heat generating elements thereby conducting printing by forming color to heat sensitive recording paper in a heat sensitive printer or partially melting an ink of an ink ribbon and transferring the same to common paper in a thermal transfer printer.
FIG. 8 shows a general thermal head of the prior art in which a temperature keeping layer 2, for example, made of glass is formed on a heat dissipating substrate 1 made of an insulative material such as alumina (hereinafter referred to as a substrate), and the temperature keeping layer 2 is formed such that the upper surface forms an arcuate shape. A plurality of heat generating resistor members 3 are linearly arranged on the top 2a of the temperature keeping layer 2 linearly in a direction perpendicular to the drawing. The heat generating resistor member 3 is formed by depositing a material for the heat generating resistor member 3, for example made of Ta—SiO2 on the surface of the temperature keeping layer 2 by means of sputtering or the like and then applying photolithographic etching. A common electrode 4a connected with each of the heat generating resistor members 3 is laminated at one side on the upper surface of the heat generating resistor member 3, while individual electrodes 4b for supplying eletric current to each of the heat generating resistor members 3 independently are laminated at the other side of each of the heat generating resistor members 3 on the upper surface of the heat generating resistor member 3 respectively. The common electrode 4a and the individual electrodes 4b are made, for example of Al and Cu and they are deposited by vapor deposition, sputtering or the like and then etched into a pattern of a desired shape.
Further, a protecting layer 5 of about 5 to 10 μm thickness is formed on the surface of the heat generating resistor members 3, the common electrode 4a, the individual electrodes 4b, and exposed surfaces of the substrate 1 and the temperature keeping layer 2, for protecting the heat generating resistor members 3 and each of the electrodes 4a, 4b. The protecting layer 5 is adapted to cover all the surface excepting for the terminal portion of each of the electrodes 4a, 4b. 
In the existent thermal head as described above, since it is necessary to lower the resistance value of the common electrode 4a by making the width greater, and as shown in FIG. 8, a heat generating portion 3a of the heat generating resistor member 3 is disposed near a central or a peripheral portion of the substrate 1 of the thermal head and a size from the heat generating portion 3a of the heat generating resistor member 3 to the end of the substrate 1 is not less than 1 mm (hereinafter referred to as an edge distance L).
However, a demand for so-called real edging of disposing the heat generating resistor member 3 of the thermal head to the end of the substrate 1 has been increased more and more in recent years, which necessities to remarkably decrease a space on the side of the common electrode 4a of the substrate 1.
The real edging of the thermal head is advantageous in that a loss of contact pressure between the head and the platen can be reduced, efficiency for printing energy can be improved and inks in a wide range from wax type to resin type can be used in a case of a thermal transfer printer using an ink ribbon, thereby remarkably improving printing quality on rough paper.
However, if it is intended for real edging of decreasing the edge distance of the thermal head, for example, to less than 0.2 mm, since the space for disposing the common electrode 4a is decreased remarkably, the lateral size of the common electrode 4a has to be made extremely small and, as a result, the common electrode 4a functions like that a resistor member to increase the resistance value thereby increasing the difference of voltage drop between both ends and the central portion of the heat generating resistor member 3. Further, it results in lack of the current capacity for the common electrode 4a to bring about a trouble such as fusion of the common electrode 4a upon current supply to each of the heat generating resistor members 3 making it extremely difficult to manufacture a real edge head of high practical usefulness.
Further, in another type of electrode for a thermal head intended for real edging, the common electrode 4a is lead in the direction identical with individual electrodes 4b, for example, in the form a turn back type or a comb-type electrode although not illustrated. However, since the common electrode 4a and the individual electrodes 4b are led out in the identical direction, identical fabrication accuracy is required for a case of resolution power at 300 dpi with that for a case of resolution power at 600 dpi and in the same manner, a fine fabrication technique is required for the resolution power at 400 dpi like that for resolution power at 800 dpi, which increases the number of production steps, lowers yield and lowers the reliability, as well as increase in the manufacturing cost.
In other type of electrode, a common electrode 4a is formed from the end face to the rear face of a substrate 1 of a thermal head. However, since the electrode is formed after dividing and polishing the substrate 1, the number of manufacturing steps is increased to lower the manufacturing efficiency, as well as this brings about a drawback that the reliability for real edging of less than 0.2 mm distance is extremely low.
In a further example of an end face edge in which a temperature keeping layer 2 is formed by polishing the end face of a substrate 1 and a heat generating resistor member 3 is formed on the upper surface thereof, the number of manufacturing steps is increased in the same manner as described above and the mass productivity is poor in a case of intending for real edging and the manufacturing cost is expensive.
Then, in the prior art system, there has been provided a coupling-type thermal head as shown in FIG. 9. Although each of thermal heads 8,8 to be connected has the same layer as that of the aforesaid prior art, its major difference consists in the fact that a common electrode terminal 9 related to an external connection of the common electrode 4a is arranged only at one side of either the right side or the left side. That is, the prior art coupling type thermal head is constructed such that the two thermal heads 8,8 having lateral symmetry shape from each other are adhered to a coupling substrate 10.
A reason why the number of connecting thermal heads in the prior art is two consists in the fact that each of the common electrode terminals 9,9 shown in FIG. 9 is formed only at one of the right side or the left side of each of the thermal heads 8,8. That is, there was no practical means for connecting more than three common electrodes of the thermal head.
In view of the above, it has been attempted to attain real edging by making the common electrode 4a into a multilayered wiring structure in a heat generating portion. In this structure, a conductive layer 6 made of a metal is formed on the temperature keeping layer 2 an interlayer insulation layer, for example, made of SiO2 is laminated thereon by means of sputtering or the like and then the interlayer insulation layer 7 is partially eliminated photolithographically, on which a heat generating resistor member 3 is stacked thereby electrically connecting the conductive layer 7 with the heat generating resistor member 3, that is, the interlayer insulation layer 7 and the conductive layer 6 are formed in a layerous structure just below the heat generating resistor member 3 that generates heat at a high temperature.
In the thermal head of the multilayered wiring structure as described above, when electric current is supplied to a desired heat generating resistor member 3 by way of an individual electrode 4b based on a desired printing signal, since electric current is supplied to the terminal portion by way of the conductive layer 6 in addition to the common electrode 4a formed at an extremely small lateral size by real edging, so that the resistance value of the common electrode 4a is not increased thereby enabling to prevent generation of a partial voltage difference in the heat generating resistor member 3, and lack of current capacity of the common electrode 4a, to attain high quality printing.
However, in the existent thermal head described above, since the interlayer insulation layer 7 is formed to the upper surface of the conductive layer 6 and each of the layers if formed just below the heat generating resistor member 3 that generates heat at high temperature, stresses between each of the layers is large and reliability for close bonding between each of the layers against thermal impacts is remarkably deteriorated. Further, since the interlayer insulation layer 7 is formed by etching, a step is caused to the surface of the interlayer insulation layer 7 and the conductive layer 6, and the step may possibly cause connection failure between the heat generating resistor member 3 and the conductive layer 6. Further, if the interlayer insulation layer 7 is formed by a vapor deposition method such as sputtering, pinholes are generated due to obstacles to the interlayer insulation layer 7 to bring about insulation failure for the interlayer insulation layer 7.